Corrections for heat equation tutorial

[lokmanb]

Just a few corrections to the heat equation tutorial:

• typo correction (issues Fix heat equation documentation #21 and Missing \rho C_p in doc #29 )
• using functions for solving steady and unsteady problems (issue Fix functions overwriting in heat_equation tutorial  #13 )
• using custom solver with linsolve
• a few changes to the documentation

Also, I propose to reduce the tolerance for the last test of the Helmholtz case to $$10^{-11}$$ because it didn't pass for the zero eigen value on my computer. I got $$9 , 10^{-13}$$ and the reference is $$-2 , 10^{-13}$$, which causes the difference to go above the $$10^{-12}$$ tolerance.

[bmxam]

The CI failed again. @lokmanb what version of julia are you running on your PC? Maybe the feature you're using is only in julia >= 1.11...